The present invention relates to the field of optically controllable microelectrode arrays for stimulating cells within a tissue. Still more specifically, the invention relates to an array having a substrate with a surface. The substrate is adapted to be placed on the tissue with the surface adjoining the cells. The substrate further comprises a plurality of electrodes on the surface in contact with the cells for stimulating same. An electrical stimulus is exerted from the electrodes to the cells under the control of light impinging on the tissue.
European published patent application 0 460 320 discloses a subretinal implant adapted to be implanted into the lower layers of the retina. The prior art implant essentially consists of a silicon chip being configured by a large number of densely packed microphotodiodes. The photoactive surface of the photodiodes is directed towards the light impinging on the eye. The photodiodes generate an amplitude-modulated current stimulating the retinal cell layer lying on the implant surface. In such a way it shall be possible to enable patients suffering from various forms of retinal degeneration to improve or even reestablish vision.
In an earlier unpublished approach it has been suggested to use a specific microelectrode array for general applications. Within this approach biological cells in networks should be electrically stimulated. For that purpose one has used a plurality of microelectrodes. Each microelectrode comprises a contact electrode being adapted to be brought in electrical contact with the network of biological cells. Further, a connecting electrode is provided that is electrically connected with a measuring instrument or the like. Finally, a light sensitive element is used located between the contact electrode and the connector electrode. A preferred field of application of such arrays is the field of retina implants, however, the earlier approach is also adapted to be used for stimulating electrogenic cells xe2x80x9cin vivoxe2x80x9d, for example in connection with cardiac pacers, muscle cell stimulators, bladder stimulators, wherein an optical light control, in particular by means of infrared light, may be effected through the skin.
In this prior approach the individual microelectrodes are optically controlled. As explained at the outset, such a structure may be used for generating electrical stimuli corresponding to image points on a retina.
Insofar it is of no importance whether the implant is a subretinal implant being implanted between lower layers of the retina or an epiretinal implant being placed on the front side of the retina.
In the prior art approaches the electrodes are configured as xe2x80x9cmicroelectrodesxe2x80x9d, however, the actual dimensions of such microelectrodes are limited by the particularly manufacturing method used.
In prior art microelectrode arrays one has used microelectrodes with a diameter in the order of 10 xcexcm. Hence, the surface portion of such microelectrodes that is available for making an electrical contact to a cell is in the same order of magnitude as the size of the cell itself.
With such configurations the cell will normally not be placed centrically on the respective electrode so as to entirely cover same. Instead, the cell will normally be offset with respect to the electrode so that only a portion of its entire surface that could be used for making contact will overlap with the corresponding contact surface of the microelectrode. However, this results in losses because the xe2x80x9cfreexe2x80x9d surface portion of the microelectrode being not in contact with the cell will emit a current into the electrolyte of the retina which, however, does not generate a stimulus but will short-circuit the microelectrode instead.
It is, therefore, an object underlying the invention to improve an array of the kind mentioned at the outset such that the energy being available for generating stimuli is almost completely transmitted to the cells of the tissue.
These and other objects of the invention are solved in that the electrodes are dimensioned such that a first surface area on the electrodes in contact with the cells is essentially smaller than a second surface area on the cells in which the cells may be contacted by the electrodes.
The object underlying the invention is thus entirely solved. If, in simple words, the microelectrodes are made substantially smaller then the cells within the tissue, it is highly unlikely that the microelectrode will only partially be covered by the cell. Instead, in most cases the much bigger cell will completely cover several of the much smaller microelectrodes. If that is the case these electrodes may transmit their stimulus exclusively and completely to the cell but not to the surrounding electrolyte.
In a preferred embodiment of the array according to the invention the ratio between the second surface area and the first surface area is between 5 and 10.
This measure has the advantage that the effect described before may be achieved for dimensions of electrodes that may be realized with manufacturing methods available today.
In a particularly preferred embodiment of the invention the electrodes are arranged on the substrate surface in the geometrically densest possible arrangement, however, electrically isolated from each other.
This measure has the advantage that the cells may come in contact with the electrodes with their entire surface. For, if the electrodes are electrically isolated from each other and are packed as dense as geometrically possible, the electrodes are configured as a layer on the array surface and have a lateral conductivity being almost zero.
In another embodiment of the invention the electrodes are formed on the substrate by way of lithography.
For particularly small dimensions of the electrodes other methods may be considered. In particular, it is preferred to manufacture the electrodes through local crystallization within a layer of the substrate. The electrodes are, for example, configured by crystal nuclei organizing themselves on the substrate surface.
Likewise various configurations of such electrodes may be manufactured in very thin amorphous layers, for example from hydrogenized silicon (a-Si:H) by combined application of metal-induced crystallization and selective etching.
If the substrate layer itself is, for example, a P-layer and the lateral conductivity is set to be very small, then the layer configured by the electrodes themselves may be utilized as a part of a semiconductor switch or a microphotodiode.
It is particularly preferred when the electrodes are a part of a light-controlled switch by means of which a voltage may be applied as a local stimulus to a cell
This measure has the advantage that the stimulating process as such may be exclusively triggered by an optical signal. This is of particular importance in connection with retina implants because the image impinging on the retina has both bright and dark portions. Hence, a switching and stimulation is effected within the bright areas whereas in the dark areas a stimulation is suppressed and the corresponding switches remain in their closed state.
As already indicated, it is preferred that the switch is configured by a plurality of substrate layers and the electrodes.
This has the advantage that the entire array is configured extremely small and thin when the layer configured by the electrodes itself is a layer of the semiconductor switch or the microphotodiode.
As already mentioned above, implants of the kind of interest may be utilized as subretinal implants or as epiretinal implants. In the latter case one must make sure that the implant itself is essentially permeable for visible light.
Further advantages will become apparent from the description and the enclosed drawing.
It goes without saying that the features specified before and those that will be explained hereinafter may not only be used in the particularly given combination but also in other combinations or alone without leaving the scope of the present invention.